The present invention relates to a dehydrated vegetable product having pre-determined quantities of solutes accurately incorporated into vegetables thereby producing a dehydrated vegetable product with relatively high moisture content and high stability. In general the dehydrated products are produced having water activities between 0.35 and 0.85 at 20.degree. C.
As is recognized in the art, water activity is a significant factor in controlling microbial growth. A discussion of water activity and its significance in this respect may be found, for instance, in Potter, "Food Science", second edition, 1973, The AVI Publishing Co., Westport, Conn., at pages 147 and 289 through 293.
The dehydrated vegetables according to the present invention are stable and have a more rapid rehydration time, improved color, flavor and texture, higher moisture content and lower production cost than dehydrated vegetables produced by known processes.
The term "vegetables" as used herein denotes all types of vegetables including the leaves, roots, bulbs and stems thereof, and fruit.
The expression "water activity controlling solute" includes but is not limited to salts (for example sodium chloride), sugars (for example sucrose, dextrose, sugar derivatives and the like), soluble protein hydrolysates, and mixtures thereof. The use of prohibited food additives is not contemplated.
Of recent years there has been a considerable increase in demand for dried packaged convenience foods such as soups, risottos and casseroles. These packaged foods normally contain dehydrated vegetables of various types, which desirably should rapidly rehydrate. For packs of good quality, it is not unusual for manufacturers to use freeze-dried vegetables. These however, are quite expensive, largely as a result of the high cost of capital equipment and high energy usage associated with their processing. They also must be maintained at a low moisture content, and if co-packed with cereals, pasta, etc., these other major ingredients must be reduced in moisture to be compatible with the dehydrated vegetables. In addition because of their need to be maintained at a low moisture content, sophisticated packaging materials must be used for their storage and distribution. A similar situation exists with conventionally hot-air dried vegetables, but these suffer additionally from slow rehydration time and poor flavor and texture.
In the normal air-drying of vegetables, approximately 75% of the moisture is removed during the first 25% of the drying time. In the final 75% of the drying time, a mere 20% of the original moisture is removed. In the final 75% of the drying time most of the flavor, texture and color loss occurs. Moreover, because of high energy input and slow throughput through capital intensive equipment such processes have proved to be expensive. With most common vegetables, maximum moisture content after dehydration is approximately 5% but good commercial practice is for the moisture content to be considerably lower and this of course prolongs the time of processing of vegetables treated in this manner.
By introducing solutes (as defined hereinbefore) into vegetables, the water activity of the dried product can be substantially reduced so that shelf-stable "intermediate moisture" dried products can be prepared. These products, when boiled with water, or when boiling water is poured on to them, rehydrate more rapidly because they start off at a higher moisture content, and also because of the presence of solutes, which induce more rapid intake of water into the vegetable tissue.
The addition of solutes to processed vegetables is not new per se. The preservation of fresh vegetables with salt has been practised for hundreds of years. More recently various methods have been published wherein salt is introduced in vegetables prior to drying by blanching the vegetables in salt solutions, or by steeping the vegetables either before or after drying in salt, sugar or other solutions. These methods have not found favour or commercial benefit because of the disadvantages encountered in employing such methods. They are awkward to use in practice. More importantly, it is very difficult to adequately control the amount of solute intake from the solutions.
In order to be of practical value such a steep solution in prior art processes that must be used more than once. If vegetable material is immersed in the steep solution, it absorbs solutes from the solution and simultaneously water enters the steep solution from the vegetables. Thus after a steeping process, the composition of the steep solution will have been markedly altered by immersion of the vegetables. Moreover, in a steep solution containing a mixture of dissolved substances, the proportion of the various ingredients absorbed by the vegetables will be different, leaving a residual steep solution with a different proportional composition than originally. It is well known, for instance, that salt (NaCl) is more rapidly absorbed from a solution by vegetables than is sucrose. It is quite impracticable in modern manufacturing practice, to have to analyse and replace into the steep solution those ingredients which have been absorbed from the solution, and it would be virtually impossible to control the contamination of the steep solution by soluble compounds absorbed into it from the vegetables.
Furthermore, during steeping the steep solution becomes contaminated with insoluble matter derived from the vegetables, and if it is reused, it becomes necessary to remove this debris by filtration, centrifugation etc. With hot steep solution, starch particles derived from the vegetables (particularly from legumes and pulses) becomes gelatinized and are difficult to remove from the solution.
Using cool or warm steep solutions, microbial contamination becomes a problem, and it becomes therefore even more difficult to reuse the solutions.
Furthermore, acids and enzymes absorbed into the solution from the vegetables or fruits can act on the sugars in the solution to hydrolyse them to reducing sugars which can cause browning of the vegetables or fruit during storage.
It becomes very difficult in practice to adequately drain vegetables which have been steeped, particularly if steeped in sugar solutions. Furthermore, the free liquid adhering to the vegetable pieces after draining causes great problems during dehydration.
Our method of introducing solutes, does not utilize a steeping step. It obviates all of the difficulties referred to in the steeping method and because of its ease and simplicity, enables high speed, efficient dehydration to be achieved.
Moreover, because the additions can be made with great exactitude it allows us to control exactly the composition of additives, which is essential to the commercial production of intermediate moisture vegetables in the range of water activities which we claim.